Both the environment and somatic mutations govern the aggregation pathway of pathogenic immunoglobulin light chain

Cu(II) binding to the λ6aJL2-R24G antibody light chain protein associated with light chain amyloidosis disease: The role of histidines

Light chain amyloidosis is a conformational disease caused by the abnormal proliferation and deposition of antibody light chains as amyloid fibers in organs and tissues. The effect of Cu(II) binding to the model recombinant protein 6aJL2-R24G was previously characterized in our group, and we found an acceleration of the aggregation kinetics of the protein. In this study, in order to confirm the Cu(II) binding sites, histidine variants of 6aJL2-R24G were prepared and the effects of their interaction with Cu(II) were analyzed by circular dichroism, fluorescence spectroscopy, isothermal calorimetry titrations, and molecular dynamics simulations. Confirming our earlier work, we found that His8 and His99 are the highest affinity Cu(II) binding sites, and that Cu(II) binding to both sites is a cooperative event.

Role of the mechanisms for antibody repertoire diversification in monoclonal light chain deposition disorders: when a friend becomes foe

The adaptive immune system of jawed vertebrates generates a highly diverse repertoire of antibodies to meet the antigenic challenges of a constantly evolving biological ecosystem. Most of the diversity is generated by two mechanisms: V(D)J gene recombination and somatic hypermutation (SHM). SHM introduces changes in the variable domain of antibodies, mostly in the regions that form the paratope, yielding antibodies with higher antigen binding affinity. However, antigen recognition is only possible if the antibody folds into a stable functional conformation. Therefore, a key force determining the survival of B cell clones undergoing somatic hypermutation is the ability of the mutated heavy and light chains to efficiently fold and assemble into a functional antibody. The antibody is the structural context where the selection of the somatic mutations occurs, and where both the heavy and light chains benefit from protective mechanisms that counteract the potentially deleterious impact of the changes. However, in patients with monoclonal gammopathies, the proliferating plasma cell clone may overproduce the light chain, which is then secreted into the bloodstream. This places the light chain out of the protective context provided by the quaternary structure of the antibody, increasing the risk of misfolding and aggregation due to destabilizing somatic mutations. Light chain-derived (AL) amyloidosis, light chain deposition disease (LCDD), Fanconi syndrome, and myeloma (cast) nephropathy are a diverse group of diseases derived from the pathologic aggregation of light chains, in which somatic mutations are recognized to play a role. In this review, we address the mechanisms by which somatic mutations promote the misfolding and pathological aggregation of the light chains, with an emphasis on AL amyloidosis. We also analyze the contribution of the variable domain (V_L) gene segments and somatic mutations on light chain cytotoxicity, organ tropism, and structure of the AL fibrils. Finally, we analyze the most recent advances in the development of computational algorithms to predict the role of somatic mutations in the cardiotoxicity of amyloidogenic light chains and discuss the challenges and perspectives that this approach faces.

Understanding AL amyloidosis with a little help from in vivo models

Monoclonal immunoglobulin (Ig) light chain amyloidosis (AL) is a rare but severe disease that may occur when a B or plasma cell clone secretes an excess of free Ig light chains (LCs). Some of these LCs tend to aggregate into organized fibrils with a β-sheet structure, the so-called amyloid fibrils, and deposit into the extracellular compartment of organs, such as the heart or kidneys, causing their dysfunction. Recent findings have confirmed that the core of the amyloid fibrils is constituted by the variable (V) domain of the LCs, but the mechanisms underlying the unfolding and aggregation of this fragment and its deposition are still unclear. Moreover, in addition to the mechanical constraints exerted by the massive accumulation of amyloid fibrils in organs, the direct toxicity of these variable domain LCs, full-length light chains, or primary amyloid precursors (oligomers) seems to play a role in the pathogenesis of the disease. Many in vitro studies have focused on these topics, but the variability of this disease, in which each LC presents unique properties, and the extent and complexity of affected organs make its study in vivo very difficult. Accordingly, several groups have focused on the development of animal models for years, with some encouraging but mostly disappointing results. In this review, we discuss the experimental models that have been used to better understand the unknowns of this pathology with an emphasis on in vivo approaches. We also focus on why reliable AL amyloidosis animal models remain so difficult to obtain and what this tells us about the pathophysiology of the disease.

Interactions of intrinsically disordered proteins with the unconventional chaperone human serum albumin: From mechanisms of amyloid inhibition to therapeutic opportunities

Human Serum Albumin (HSA), the most abundant protein in plasma, serves a diverse repertoire of biological functions including regulation of oncotic pressure and redox potential, transport of serum solutes, but also chaperoning of misfolded proteins. Here we review how HSA interacts with a wide spectrum of client proteins including intrinsically disordered proteins (IDPs) such as Aβ, the islet amyloid peptide (IAPP), alpha synuclein and stressed globular proteins such as insulin. The comparative analysis of the HSA chaperone - client interactions reveals that the amyloid-inhibitory function of HSA arises from at least four emerging mechanisms. Two mechanisms (the monomer stabilizer model and the monomer competitor model) involve the direct binding of HSA to either IDP monomers or oligomers, while other mechanisms (metal chelation and membrane protection) rely on the indirect modulation by HSA of other factors that drive IDP aggregation. While HSA is not the only extracellular chaperone, given its abundance, HSA is likely to account for a significant fraction of the chaperoning effects in plasma, thus opening new therapeutic opportunities in the context of the peripheral sink hypothesis.

Machine learning analyses of antibody somatic mutations predict immunoglobulin light chain toxicity

In systemic light chain amyloidosis (AL), pathogenic monoclonal immunoglobulin light chains (LC) form toxic aggregates and amyloid fibrils in target organs. Prompt diagnosis is crucial to avoid permanent organ damage, but delayed diagnosis is common because symptoms usually appear only after strong organ involvement. Here we present LICTOR, a machine learning approach predicting LC toxicity in AL, based on the distribution of somatic mutations acquired during clonal selection. LICTOR achieves a specificity and a sensitivity of 0.82 and 0.76, respectively, with an area under the receiver operating characteristic curve (AUC) of 0.87. Tested on an independent set of 12 LCs sequences with known clinical phenotypes, LICTOR achieves a prediction accuracy of 83%. Furthermore, we are able to abolish the toxic phenotype of an LC by in silico reverting two germline-specific somatic mutations identified by LICTOR, and by experimentally assessing the loss of in vivo toxicity in a Caenorhabditis elegans model. Therefore, LICTOR represents a promising strategy for AL diagnosis and reducing high mortality rates in AL.

Understanding Mesangial Pathobiology in AL-Amyloidosis and Monoclonal Ig Light Chain Deposition Disease

Patients with plasma cell dyscrasias produce free abnormal monoclonal Ig light chains that circulate in the blood stream. Some of them, termed glomerulopathic light chains, interact with the mesangial cells and trigger, in a manner dependent of their structural and physicochemical properties, a sequence of pathological events that results in either light chain–derived (AL) amyloidosis (AL-Am) or light chain deposition disease (LCDD). The mesangial cells play a key role in the pathogenesis of both diseases. The interaction with the pathogenic light chain elicits specific cellular processes, which include apoptosis, phenotype transformation, and secretion of extracellular matrix components and metalloproteinases. Monoclonal light chains associated with AL-Am but not those producing LCDD are avidly endocytosed by mesangial cells and delivered to the mature lysosomal compartment where amyloid fibrils are formed. Light chains from patients with LCDD exert their pathogenic signaling effect at the cell surface of mesangial cells. These events are generic mesangial responses to a variety of adverse stimuli, and they are similar to those characterizing other more frequent glomerulopathies responsible for many cases of end-stage renal disease. The pathophysiologic events that have been elucidated allow to propose future therapeutic approaches aimed at preventing, stopping, ameliorating, or reversing the adverse effects resulting from the interactions between glomerulopathic light chains and mesangium.

Site-Specific Interactions with Copper Promote Amyloid Fibril Formation for λ6aJL2-R24G

Light-chain amyloidosis (AL) is one of the most common systemic amyloidoses, and it is characterized by the deposition of immunoglobulin light chain (LC) variable domains as insoluble amyloid fibers in vital organs and tissues. The recombinant protein 6aJL2-R24G contains λ6a and JL2 germline genes and also contains the Arg24 by Gly substitution. This mutation is present in 25% of all amyloid-associated λ6 LC cases, reduces protein stability, and increases the propensity to form amyloid fibers. In this study, it was found that the interaction of 6aJL2-R24G with Cu(II) decreases the thermal stability of the protein and accelerates the amyloid fibril formation, as observed by fluorescence spectroscopy. Isothermal calorimetry titration showed that Cu(II) binds to the protein with micromolar affinity. His99 may be one of the main Cu(II) interaction sites, as observed by nuclear magnetic resonance spectroscopy. The binding of Cu(II) to His99 induces larger fluctuations of the CDR1 and loop C″, as shown by molecular dynamics simulations. Thus, Cu(II) binding may be inducing the loss of interactions between CDR3 and CDR1, making the protein less stable and more prone to form amyloid fibers. This study provides insights into the mechanism of metal-induced aggregation of the 6aJL2-R24G protein and sheds light on the bio-inorganic understanding of AL disease.

The CDR1 and Other Regions of Immunoglobulin Light Chains are Hot Spots for Amyloid Aggregation

Immunoglobulin light chain-derived (AL) amyloidosis is a debilitating disease without known cure. Almost nothing is known about the structural factors driving the amyloidogenesis of the light chains. This study aimed to identify the fibrillogenic hotspots of the model protein 6aJL2 and in pursuing this goal, two complementary approaches were applied. One of them was based on several web-based computational tools optimized to predict fibrillogenic/aggregation-prone sequences based on different structural and biophysical properties of the polypeptide chain. Then, the predictions were confirmed with an ad-hoc synthetic peptide library. In the second approach, 6aJL2 protein was proteolyzed with trypsin, and the products incubated in aggregation-promoting conditions. Then, the aggregation-prone fragments were identified by combining standard proteomic methods, and the results validated with a set of synthetic peptides with the sequence of the tryptic fragments. Both strategies coincided to identify a fibrillogenic hotspot located at the CDR1 and β-strand C of the protein, which was confirmed by scanning proline mutagenesis analysis. However, only the proteolysis-based strategy revealed additional fibrillogenic hotspots in two other regions of the protein. It was shown that a fibrillogenic hotspot associated to the CDR1 is also encoded by several κ and λ germline variable domain gene segments. Some parts of this study have been included in the chapter “The Structural Determinants of the Immunoglobulin Light Chain Amyloid Aggregation”, published in Physical Biology of Proteins and Peptides, Springer 2015 (ISBN 978-3-319-21687-4).

Influence of a Single Point Mutation in the Constant Domain of the Bence-Jones Protein bif on Its Aggregation Properties

Multiple myeloma nephropathy occurs due to the aggregate formation by monoclonal immunoglobulin light chains (Bence-Jones proteins) in kidneys of patients with multiple myeloma. The mechanism of amyloid deposit formation is still unclear. Earlier, the key role in the fibril formation has been assigned to the variable domains that acquired amyloidogenic properties as a result of somatic mutations. However, fibril formation by the Bence-Jones protein BIF was found to be the function of its constant domain. The substitution of Ser177 by Asn in the constant domain of the BIF protein is most likely an inherited than a somatic mutation. To study the role of this mutation in amyloidogenesis, the recombinant Bence-Jones protein BIF and its mutant with the N177S substitution typical for the known immunoglobulin C_κ allotypes Km¹, Km^1,2, and Km³ were isolated. The morphology of aggregates formed by the recombinant proteins under conditions similar to those occurring during the protein transport in bloodstream and its filtration into the renal glomerulus, in the distal tubules, and in the proximal renal tubules was analyzed by atomic force microscopy. The nature of the aggregates formed by BIF and its N177S mutant during incubation for 14 days at 37°C strongly differed and depended on both pH and the presence of a reducing agent. BIF formed fibrils at pH 7.2, 6.5, and 10.1, while the N177S mutant formed fibrils only at alkaline pH 10.1. The refolding of both proteins in the presence of 5 mM dithiothreitol resulted in the formation of branched structures.

Stability and aggregation propensity do not fully account for the association of various germline variable domain gene segments with light chain amyloidosis

Variable domain (VL) gene segments exhibit variable tendencies to be associated with light chain amyloidosis (AL). While few of them are very frequent in AL and give rise to most of the amyloidogenic light chains compiled at the sequence databases, other are rarely found among the AL cases. To analyze to which extent these tendencies depend on folding stability and aggregation propensity of the germline VL protein, we characterized VL proteins encoded by four AL-associated germline gene segments and one not associated to AL. We found that the AL-associated germline rVL proteins differ widely in conformational stability and propensity to in vitro amyloid aggregation. While in vitro the amyloid formation kinetics of these proteins correlate well with their folding stabilities, the folding stability does not clearly correlate with their germline's frequencies in AL. We conclude that the association of the VL genes segments to amyloidosis is not determined solely by the folding stability and aggregation propensity of the germline VL protein. Other factors, such as the frequencies of destabilizing mutations and susceptibility to proteolysis, must play a role in determining the light chain amyloidogenicity.

Cardiac Light Chain Amyloidosis: The Role of Metal Ions in Oxidative Stress and Mitochondrial Damage

AIMS

The knowledge of the mechanism underlying the cardiac damage in immunoglobulin light chain (LC) amyloidosis (AL) is essential to develop novel therapies and improve patients' outcome. Although an active role of reactive oxygen species (ROS) in LC-induced cardiotoxicity has already been envisaged, the actual mechanisms behind their generation remain elusive. This study was aimed at further dissecting the action of ROS generated by cardiotoxic LC in vivo and investigating whether transition metal ions are involved in this process. In the absence of reliable vertebrate model of AL, we used the nematode Caenorhabditis elegans, whose pharynx is an "ancestral heart."

RESULTS

LC purified from patients with severe cardiac involvement intrinsically generated high levels of ROS and when administered to C. elegans induced ROS production, activation of the DAF-16/forkhead transcription factor (FOXO) pathway, and expression of proteins involved in stress resistance and survival. Profound functional and structural ROS-mediated mitochondrial damage, similar to that observed in amyloid-affected hearts from AL patients, was observed. All these effects were entirely dependent on the presence of metal ions since addition of metal chelator or metal-binding 8-hydroxyquinoline compounds (chelex, PBT2, and clioquinol) permanently blocked the ROS production and prevented the cardiotoxic effects of amyloid LC. Innovation and Conclusion: Our findings identify the key role of metal ions in driving the ROS-mediated toxic effects of LC. This is a novel conceptual advance that paves the way for new pharmacological strategies aimed at not only counteracting but also totally inhibiting the vicious cycle of redox damage. Antioxid. Redox Signal. 27, 567-582.

Effect of amino acid mutations on the conformational dynamics of amyloidogenic immunoglobulin light-chains: A combined NMR and in silico study

The conformational dynamics of a pathogenic κ4 human immunoglobulin light-chain variable domain, SMA, associated with AL amyloidosis, were investigated by ¹⁵N relaxation dispersion NMR spectroscopy. Compared to a homologous light-chain, LEN, which differs from SMA at eight positions but is non-amyloidogenic in vivo, we find that multiple residues in SMA clustered around the N-terminus and CDR loops experience considerable conformational exchange broadening caused by millisecond timescale protein motions, consistent with a destabilized dimer interface. To evaluate the contribution of each amino acid substitution to shaping the dynamic conformational landscape of SMA, NMR studies were performed for each SMA-like point mutant of LEN followed by in silico analysis for a subset of these proteins. These studies show that a combination of only three mutations located within or directly adjacent to CDR3 loop at the dimer interface, which remarkably include both destabilizing (Q89H and Y96Q) and stabilizing (T94H) mutations, largely accounts for the differences in conformational flexibility between LEN and SMA. Collectively, our studies indicate that a correct combination of stabilizing and destabilizing mutations is key for immunoglobulin light-chains populating unfolded intermediates that result in amyloid formation, and underscore the complex nature of correlations between light-chain conformational flexibility, thermodynamic stability and amyloidogenicity.

Mutations can cause light chains to be too stable or too unstable to form amyloid fibrils

Light chain (AL) amyloidosis is an incurable human disease, where the amyloid precursor is a misfolding-prone immunoglobulin light-chain. Here, we identify the role of somatic mutations in the structure, stability and in vitro fibril formation for an amyloidogenic AL-12 protein by restoring four nonconservative mutations to their germline (wild-type) sequence. The single restorative mutations do not affect significantly the native structure, the unfolding pathway, and the reversibility of the protein. However, certain mutations either decrease (H32Y and H70D) or increase (R65S and Q96Y) the protein thermal stability. Interestingly, the most and the least stable mutants, Q96Y and H32Y, do not form amyloid fibrils under physiological conditions. Thus, Q96 and H32 are key residues for AL-12 stability and fibril formation and restoring them to the wild-type residues preclude amyloid formation. The mutants whose equilibrium is shifted to either the native or unfolded states barely sample transient partially folded states, and therefore do not form fibrils. These results agree with previous observations by our laboratory and others that amyloid formation occurs because of the sampling of partially folded states found within the unfolding transition (Blancas-Mejia and Ramirez-Alvarado, Ann Rev Biochem 2013;82:745-774). Here we provide a new insight on the AL amyloidosis mechanism by demonstrating that AL-12 does not follow the established thermodynamic hypothesis of amyloid formation. In this hypothesis, thermodynamically unstable proteins are more prone to amyloid formation. Here we show that within a thermal stability range, the most stable protein in this study is the most amyloidogenic protein.

New aspects on the pathogenesis of renal disorders related to monoclonal gammopathies

BACKGROUND

Multiple myeloma and other related monoclonal gammopathies are frequently encountered conditions associated with renal damage, especially in elderly population. They are arising from clonal proliferation of plasma cells in bone marrow producing various quantities of abnormal monoclonal immunoglobulins, or their components/fragments.

SUMMARY

These abnormal proteins differ from normal immunoglobulins in the amino acid sequence and in the three-dimensional structure of the molecule, which may determine their toxicity. Kidney seems to be a target organ as a major catabolic site. The pathology of renal disease is highly heterogeneous involving a variety of different mechanisms, which are divided into immunoglobulin dependent and immunoglobulin independent mechanisms. The Ig-dependent mechanisms may involve the four components of the kidney parenchyma, and the primary structure of these proteins determine the pattern of renal disease.

KEY MESSAGE

This review summarizes the existing literature in the pathobiology of multiple myeloma, and the pathological properties of the M-proteins, focusing on the mechanisms of the renal manifestations related to these abnormal proteins, especially glomerular injury. Also it supports the opinion that monoclonal gammopathy of undetermined significance (MGUS) should not be used in cases where there is proven renal impairment due to these proteins, even if it is mild and does not meet the current criteria.

Monoclonal light chain deposits within the stomach manifesting as immunotactoid gastropathy

Immunotactoid deposits are defined by their ultrastructural appearance and are characterized by microtubular or cylindrical structures typically measuring greater than 30 nm in diameter. Although a rare entity, immunotactoid deposition most often manifests as immunotactoid glomerulopathy and is associated with underlying lymphoplasmacytic disorders. Corneal immunotactoid deposition known as immunotactoid keratopathy has also been reported in patients with paraproteinemia. Here, we describe the first reported case of immunotactoid deposition in the stomach. The deposits were composed solely of kappa immunoglobulin light chains without significant lambda light chain or immunoglobulin heavy chain components. The patient displayed no renal signs or symptoms, and additional thorough clinical examination failed to detect any evidence of a paraproteinemia or plasma cell dyscrasia. Thus, the gastric immunotactoid deposits in this case appear to be an isolated finding of light chain deposition, of which the significance and etiology are unclear.

Unique effect of Cu(II) in the metal-induced amyloid formation of β-2-microglobulin

β-2-Microglobulin (β2m) forms amyloid fibrils in the joints of patients undergoing hemodialysis treatment as a result of kidney failure. In the presence of stoichiometric amounts of Cu(II), β2m self-associates into discrete oligomeric species, including dimers, tetramers, and hexamers, before ultimately forming amyloid fibrils that contain no copper. To improve our understanding of whether Cu(II) is unique in its ability to induce β2m amyloid formation and to delineate the coordinative interactions that allow Cu(II) to exert its effect, we have examined the binding of Ni(II) and Zn(II) to β2m and the resulting influence that these metals have on β2m aggregation. We find that, in contrast to Cu(II), Ni(II) does not induce the oligomerization or aggregation of β2m, while Zn(II) promotes oligomerization but not amyloid fibril formation. Using X-ray absorption spectroscopy and new mass spectrometry-related techniques, we find that different binding modes are responsible for the different effects of Ni(II) and Zn(II). By comparing the binding modes of Cu(II) with Ni(II), we find that Cu(II) binding to Asp59 and the backbone amide between the first two residues of β2m are important for allowing the formation of amyloid-competent oligomers, as Ni(II) appears not to bind these sites on the protein. The oligomers formed in the presence of Zn(II) are permitted by this metal's ability to bridge two β2m units via His51. These oligomers, however, are not able to progress to form amyloid fibrils because Zn(II) does not induce the required structural changes near the N-terminus and His31.

Formation of assemblies on cell membranes by secreted proteins: molecular studies of free λ light chain aggregates found on the surface of myeloma cells

We have described the presence of cell-membrane-associated κFLCs (free immunoglobulin light chains) on the surface of myeloma cells. Notably, the anti-κFLC mAb (monoclonal antibody) MDX-1097 is being assessed in clinical trials as a therapy for κ light chain isotype multiple myeloma. Despite the clinical potential of anti-FLC mAbs, there have been limited studies on characterizing membrane-associated FLCs at a molecular level. Furthermore, it is not known whether λFLCs can associate with cell membranes of myeloma cells. In the present paper, we describe the presence of λFLCs on the surface of myeloma cells. We found that cell-surface-associated λFLCs are bound directly to the membrane and in an aggregated form. Subsequently, membrane interaction studies revealed that λFLCs interact with saturated zwitterionic lipids such as phosphatidylcholine and phosphatidylethanolamine, and using automated docking, we characterize a potential recognition site for these lipids. Atomic force microscopy confirmed that membrane-associated λFLCs are aggregated. Given the present findings, we propose a model whereby individual FLCs show modest affinity for zwitterionic lipids, with aggregation stabilizing the interaction due to multivalency. Notably, this is the first study to image FLCs bound to phospholipids and provides important insights into the possible mechanisms of membrane association by this unique myeloma surface antigen.

How early studies on secreted and membrane protein quality control gave rise to the ER associated degradation (ERAD) pathway: the early history of ERAD

All newly synthesized proteins are subject to quality control check-points, which prevent aberrant polypeptides from harming the cell. For proteins that ultimately reside in the cytoplasm, components that also reside in the cytoplasm were known for many years to mediate quality control. Early biochemical and genetic data indicated that misfolded proteins were selected by molecular chaperones and then targeted to the proteasome (in eukaryotes) or to proteasome-like particles (in bacteria) for degradation. What was less clear was how secreted and integral membrane proteins, which in eukaryotes enter the endoplasmic reticulum (ER), were subject to quality control decisions. In this review, we highlight early studies that ultimately led to the discovery that secreted and integral membrane proteins also utilize several components that constitute the cytoplasmic quality control machinery. This component of the cellular quality control pathway is known as ER associated degradation, or ERAD. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Systemic amyloidoses

The amyloidoses are a group of protein misfolding diseases in which the precursor protein undergoes a conformational change that triggers the formation of amyloid fibrils in different tissues and organs, causing cell death and organ failure. Amyloidoses can be either localized or systemic. In localized amyloidosis, amyloid deposits form at the site of precursor protein synthesis, whereas in systemic amyloidosis, amyloid deposition occurs distant from the site of precursor protein secretion. We review the type of proteins and cells involved and what is known about the complex pathophysiology of these diseases. We focus on light chain amyloidosis to illustrate how biochemical and biophysical studies have led to a deeper understanding of the pathogenesis of this devastating disease. We also review current cellular, tissue, and animal models and discuss the challenges and opportunities for future studies of the systemic amyloidoses.

Quantitative analysis of deamidation and isomerization in β2-microglobulin by 18O labeling

Deamidation of asparagine residues in proteins via the formation of a 5-membered succinimide ring intermediate is a nonenzymatic intramolecular reaction and, in general, occurs most rapidly at an Asn-Gly sequence. A protein containing this sequence would, therefore, be susceptible to modification, and the result would produce a structural alteration in the molecule. An Asn would be replaced with an Asp, resulting in an increase in the overall negative charge on the molecule but also an isomerization to isoAsp. Despite the fact that such a structural replacement could affect the functional properties of a protein, estimating the susceptibility of the Asn-Gly sequence to deamidation/isomerization remains a difficult task. This is especially true for proteins that are subjected to enzymatic digestion during their characterization, since the above transformation could occur spontaneously during this treatment. To address this issue, we applied a stable-isotope (18)O-labeling method combined with nano-LC-MS/MS to examine the susceptibility of two Asn-Gly sites in β2-microglobulin (β2m) to the reaction. The method permits the reaction occurring in a protein to be distinguished from that during enzymatic treatment. When β2m was incubated for 60 days at 37 °C, deamidation at Asn17-Gly and Asn42-Gly with half-lives of 33 and 347 days occurred, respectively. Moreover, a comparison of the deamidated products to synthetic peptides revealed that 44% of the Asp17 and 96% of the Asp42 had been converted into isoAsp forms. Interestingly, such structurally altered β2m showed a specific affinity for divalent Cu(2+) ions, which is thought to be a candidate for initiating fibril formation.

Serum albumin prevents protein aggregation and amyloid formation and retains chaperone-like activity in the presence of physiological ligands

Although serum albumin has an established function as a transport protein, evidence is emerging that serum albumin may also have a role as a molecular chaperone. Using established techniques to characterize chaperone interactions, this study demonstrates that bovine serum albumin: 1) preferentially binds stressed over unstressed client proteins; 2) forms stable, soluble, high molecular weight complexes with stressed client proteins; 3) reduces the aggregation of client proteins when it is present at physiological levels; and 4) inhibits amyloid formation by both WT and L55P transthyretin. Although the antiaggregatory effect of serum albumin is maintained in the presence of physiological levels of Ca(2+) and Cu(2+), the presence of free fatty acids significantly alters this activity: stabilizing serum albumin at normal levels but diminishing chaperone-like activity at high concentrations. Moreover, here it is shown that depletion of albumin from human plasma leads to a significant increase in aggregation under physiologically relevant heat and shear stresses. This study demonstrates that serum albumin possesses chaperone-like properties and that this activity is maintained under a number of physiologically relevant conditions.

β(2)-microglobulin amyloidosis

Dialysis-related amyloidosis (DRA) is a clinical syndrome of pain, loss of function and other symptoms due to the deposition of amyloid consisting of β(2)-microglobulin (β(2)m) in the musculoskeletal system. The condition is seen in patients who suffer from chronic kidney disease and are treated with hemodialysis for a long time. Even though β(2)m easily can be manipulated to form amyloid in laboratory experiments under non-physiological conditions the precise mechanisms involved in the formation of β(2)m-amyloid in patients with DRA have been difficult to unravel. The current knowledge which is reviewed here indicates that conformational fluctuations centered around the D-strand, the DE-loop, and around the cis-configured Pro32 peptide bond are involved in β(2)m amyloidosis. Also required are highly increased concentrations of circulating β(2)m and possibly various post-translational modifications mediated by the pro-inflammatory environment in uremic blood, together with the influence of divalent metal ions (specifically Cu(2 +)), uremic toxins, and dialysis-enhanced redox-processes. It seems plausible that domain-swapped β(2)m dimers act as building blocks of β-spine cross-β -sheet fibrils consisting of otherwise globular, roughly natively folded protein. An activated complement system and cellular activation perpetuate these reactions which due to the affinity of β(2)m-amyloid for the collagen of synovial surfaces result in the DRA syndrome.

Structural insights into the pre-amyloid tetramer of β-2-microglobulin from covalent labeling and mass spectrometry

The main pathogenic process underlying dialysis-related amyloidosis is the accumulation of β-2-microglobulin (β2m) as amyloid fibrils in the musculoskeletal system, and some evidence suggests that Cu(II) may play a role in β2m amyloid formation. Cu(II)-induced β2m fibril formation is preceded by the formation of discrete, oligomeric intermediates, including dimers, tetramers, and hexamers. In this work, we use selective covalent labeling reactions combined with mass spectrometry to investigate the amino acids responsible for mediating tetramer formation in wild-type β2m. By comparing the labeling patterns of the monomer, dimer, and tetramer, we find evidence that the tetramer interface is formed by the interaction of D strands from one dimer unit and G strands from another dimer unit. These covalent labeling data along with molecular dynamics calculations allow the construction of a tetramer model that indicates how the protein might proceed to form even higher-order oligomers.

Novel Zn2+-binding sites in human transthyretin: implications for amyloidogenesis and retinol-binding protein recognition

Human transthyretin (TTR) is a homotetrameric protein involved in several amyloidoses. Zn(2+) enhances TTR aggregation in vitro, and is a component of ex vivo TTR amyloid fibrils. We report the first crystal structure of human TTR in complex with Zn(2+) at pH 4.6-7.5. All four structures reveal three tetra-coordinated Zn(2+)-binding sites (ZBS 1-3) per monomer, plus a fourth site (ZBS 4) involving amino acid residues from a symmetry-related tetramer that is not visible in solution by NMR. Zn(2+) binding perturbs loop E-α-helix-loop F, the region involved in holo-retinol-binding protein (holo-RBP) recognition, mainly at acidic pH; TTR affinity for holo-RBP decreases ∼5-fold in the presence of Zn(2+). Interestingly, this same region is disrupted in the crystal structure of the amyloidogenic intermediate of TTR formed at acidic pH in the absence of Zn(2+). HNCO and HNCA experiments performed in solution at pH 7.5 revealed that upon Zn(2+) binding, although the α-helix persists, there are perturbations in the resonances of the residues that flank this region, suggesting an increase in structural flexibility. While stability of the monomer of TTR decreases in the presence of Zn(2+), which is consistent with the tertiary structural perturbation provoked by Zn(2+) binding, tetramer stability is only marginally affected by Zn(2+). These data highlight structural and functional roles of Zn(2+) in TTR-related amyloidoses, as well as in holo-RBP recognition and vitamin A homeostasis.

Rheumatoid arthritis and renal light-chain deposition disease: long-term effectiveness of TNF-α blockade with etanercept

A 68-year-old woman diagnosed with erosive rheumatoid arthritis (RA) was treated with intramuscular methotrexate 15 mg weekly and oral prednisone 5 mg daily. A favorable outcome of 6 years was followed by RA flare and nephrotic syndrome (NS). Renal biopsy revealed non-amyloid light-chain deposition disease. Laboratory analysis and bone marrow biopsy excluded monoclonal protein and plasma cell dyscrasia. Addition of subcutaneous etanercept, 25 mg twice weekly allowed rapid control of both arthritis and NS. To date, after over 7-year follow-up, RA is in clinical remission, 24-h albuminuria is consistently below 0.5 g, and serum creatinine is 0.9 mg/dl.

Structure of the preamyloid dimer of beta-2-microglobulin from covalent labeling and mass spectrometry

Beta-2-microglobulin (beta2m) self-associates into fibrillar amyloid deposits in the musculoskeletal system of patients undergoing hemodialysis treatment. Previous studies have shown that stoichiometric amounts of Cu(II) at near physiological conditions can cause beta2m to organize into native-like dimers prior to forming amyloid fibrils. Here, we report the results from selective covalent labeling reactions combined with mass spectrometry that provide insight into the amino acid residues that mediate dimer formation in the wild-type protein. Using three complementary covalent labeling reagents, we find that the dimer interface is formed by the antiparallel stacking of ABED beta-sheets from two beta2m monomers. In addition, our data clearly indicate that a dimer interface involving the interactions of D-D strands from separate protein units as seen in the recent crystal structures of two mutant beta2m oligomers is unlikely.

Copper binding to beta-2-microglobulin and its pre-amyloid oligomers

Beta-2-microglobulin (beta2m) deposits as amyloid fibrils in the musculoskeletal system of patients undergoing long-term dialysis treatment as a result of kidney failure. Previous work has shown that Cu(II) binding causes beta2m to organize into nativelike dimers and tetramers that precede amyloid formation. Cu(II) is then released from higher-order oligomers before mature Cu(II)-free amyloid fibrils are formed. While some of the Cu(II)-induced structural changes that enable beta2m self-assembly are starting to be revealed, the details of how the Cu(II) binding site evolves from the monomer to the dimers and tetramers are not known. Here, we report results from three mass spectrometry (MS)-based methods that provide insight into the changing Cu-beta2m interactions. We find that monomeric beta2m binds Cu(II) via the N-terminal amine, the amide of Gln2, His31, and Asp59. In the dimer and tetramer, Asp59 is no longer bound to Cu(II), but the other residues still comprise a well-defined albeit weaker binding site that is better able to release Cu(II). Consistent with this is the observation that a fraction of the tetrameric species no longer binds Cu(II) at this weakened binding site, which agrees with a previous report that suggested the tetramer as the first Cu(II)-free oligomer. Our results also provide some insight into structural changes caused by Cu(II) binding that facilitate oligomer formation. Specifically, binding by Asp59 in the monomer requires significant movement of this residue, and we propose that this repositioning is important for establishing a pair of dimer-stabilizing salt bridges between this residue and Lys19. We also find evidence that Cu(II) binding in the N-terminal region of the monomer repels Arg3, which likely allows this residue to form a pair of dimer-stabilizing salt bridges with Glu16. Overall, our measurements suggest that the previously proposed conformational switch caused by Cu(II) binding includes not only a cis-trans isomerization at Pro32 but also the repositioning of residues that are critical for the formation of new electrostatic interactions.

Metal binding sheds light on mechanisms of amyloid assembly

Beta-2 microglobulin (beta2m) is the protein responsible for amyloid deposition in Dialysis-Related Amyloidosis (DRA). Aggregation can be induced by various solution conditions including exposure to divalent metal, incubation at acidic pH, and limited proteolysis. Using Cu(2+) as a trigger, we have trapped, isolated, and crystallized a stable oligomer of beta2m that is populated under amyloidogenic solution conditions (Calabrese et al. Nat Struct Mol Biol 2008; 15:965-71). This structure reveals that Cu(2+)-binding is associated with dramatic conformational rearrangements. This has allowed us to postulate a set of structural changes common to all beta2m aggregation pathways. Cu(2+) serves as a potential trigger in other aggregation systems such as Abeta, alpha-synuclein, and mammalian Prion (PrP). A comparison of Cu(2+) binding to beta2m and PrP reveals common features. Therefore, in addition to providing insight into DRA, induction of structure by Cu(2+) binding appears to be a recurring structural motif for pathological changes in conformation.

The effect of membranes on the in vitro fibrillation of an amyloidogenic light-chain variable-domain SMA

Light chain (or AL) amyloidosis is the most common form of systemic amyloidosis, characterized by the pathological deposition of insoluble fibrils of immunoglobulin light-chain fragments in various organs and tissues, especially in the kidney and heart. Both the triggering factors and the mechanisms involved in the abnormal formation of the insoluble fibrillar aggregates from the soluble proteins are poorly understood. For example, although the fibrillar deposits are typically found associated with the extracellular matrix and basement membranes, it is not clear whether fibrils are initially formed intra- or extracellularly, nor it is understood what determines where the deposits will occur; i.e., site tropism. In the present investigation, we studied the interaction of a recombinant amyloidogenic light-chain variable domain, SMA, with lipid vesicles. The nature of the interaction was dependent on the lipid composition and the SMA to lipid ratio. The most pronounced effect was found from vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate, which dramatically accelerated fibril growth. Interestingly, spectral probes, such as intrinsic fluorescence and far-UV CD spectroscopy did not show significant conformational changes in the presence of the vesicles. The presence of cholesterol or divalent cations, such as Ca(2+) and Mg(2+), lead to decreased membrane-induced SMA fibrillation. Thus, membranes may have significant effects on light-chain fibrillation and may contribute to the site selectivity observed in AL amyloidosis.

Cu(II) organizes beta-2-microglobulin oligomers but is released upon amyloid formation

beta-2-Microglobulin (beta2m) is deposited as amyloid fibrils in the bones and joints of patients undergoing long-term dialysis treatment as a result of kidney failure. Previous work has shown that biologically relevant amounts of Cu(II) can cause beta2m to be converted to amyloid fibrils under physiological conditions in vitro. In this work, dynamic light scattering, mass spectrometry, and size-exclusion chromatography are used to characterize the role that Cu plays in the formation of oligomeric intermediates that precede fibril formation. Cu(II) is found to be necessary for the stability of the dimer and an initial form of the tetramer. The initially formed tetramer then undergoes a structural change to a state that no longer binds Cu(II) before progressing to a hexameric state. Based on these results, we propose that the lag phase associated with beta2m fibril formation is partially accounted for by the structural transition of the tetramer that results in Cu(II) loss. Consistent with this observation is the determination that the mature beta2m amyloid fibrils do not contain Cu. Thus, Cu(II) appears to play a catalytic role by enabling the organization of the necessary oligomeric intermediates that precede beta2m amyloid formation.

Characterization of the Binding of Cu(II) and Zn(II) to Transthyretin: Effects on Amyloid Formation

Although metal ions can promote amyloid formation from many proteins, their effects on the formation of amyloid from transthyretin have not been previously studied. We therefore screened the effects of Cu(II), Zn(II), Al(III), and Fe(III) on amyloid formation from wild-type (WT) transthyretin as well as its V30M, L55P, and T119M mutants. Cu(II) and Zn(II) promoted amyloid formation from the L55P mutant of transthyretin at pH 6.5 but had little effect on amyloid formation from the other forms of the protein. Zn(II) promoted L55P amyloid formation at pH 7.4 but Cu(II) inhibited it. Cu(II) gave dose-dependent quenching of the tryptophan fluorescence of transthyretin and the fluorescence of 1-anilino-8-naphthalene sulfonate bound to it. Zn(II) gave dose-dependent quenching of the tryptophan but not the 1-anilino-8-naphthalene sulfonate fluorescence. Apparent dissociation constants for Cu(II) and Zn(II) binding at pH 7.4 of approximately 10 nM and approximately 1 microM (approximately 0.4 microM and approximately 5 microM at pH 6.5), respectively, were obtained from the quenching data. Zn(II) enhanced urea-mediated the dissociation of the L55P but not the WT transthyretin tetramer. Cu(II), depending on its concentration, either had no effect or stabilized the WT tetramer but could enhance urea-mediated dissociation of L55P.

Formation of a Stable Oligomer of β-2 Microglobulin Requires only Transient Encounter with Cu(II)

Beta-2 Microglobulin (beta2m) is a small, globular protein, with high solubility under conditions comparable to human serum. A complication of hemodialysis in renal failure patients is the deposition of unmodified beta2m as amyloid fibers. In vitro, exposure of beta2m to equimolar Cu(2+) under near-physiological conditions can result in self-association leading to amyloid fiber formation. Previously, we have shown that the early steps in this process involve a catalyzed structural rearrangement followed by formation of discrete oligomers. These oligomers, however, have a continued requirement for Cu(2+) while mature fibers are resistant to addition of metal chelate. Here, we report that the transition from Cu(2+) dependent to chelate resistant states occurs in the context of small oligomers, dimeric to hexameric in size. These species require Cu(2+) to form, but once generated, do not need metal cation for stability. Importantly, this transition occurs gradually over several days and the resulting oligomers are isolatable and kinetically stable on timescales exceeding weeks. In addition, formation is enhanced by levels of urea similar to those found in hemodialysis patients. Our results are consistent with our hypothesis that transient encounter of full-length wild-type beta2m with transition metal cation at the dialysis membrane interface is causal to dialysis related amyloidosis.

A 19-year-old male with generalized seizures, unconsciousness and a deviation of gaze

Light chain deposition disease (LCDD) is a form of monoclonal immunoglobulin deposition diseases (MIDD) which in contrast to light-chain derived (AL) amyloidosis is characterized by non-congophilic, non-fibrillary monoclonal protein deposits. Systemic organ deposits are common with the kidney being a major target organ. A clonal lymphoplasmocytic proliferation, e.g. plasmacytoma, is present in the majority of cases. Here we report on a 19-year-old male who presented with generalized seizures and an enhancing white matter lesion on MRI scans. A stereotactic brain biopsy revealed a low-grade B cell lymphoma with plasmacellular differentiation as well as lambda light chain deposits without birefringence under polarized microscopy. No systemic lymphoma manifestations or systemic light chain deposits were found, nor was a monoclonal gammopathy detectable in serum and urine. After systemic chemotherapy with three courses high-dose methotrexate the size of the lesion and the condition of the patient have remained stable for 24 months now. This is the first description of cerebral LCDD developing without systemic disease in conjunction with the diagnosis of a cerebral low-grade B cell lymphoma. We present the clinical, laboratory and radiological findings and discuss the pathogenesis of this unusual LCDD manifestation.

Scarcity of lambda 1 B cells in mice with a single point mutation in C lambda 1 is due to a low BCR signal caused by misfolded lambda 1 light chain

The presence of valine-154 instead of glycine in the constant region of lambda1 causes a severe lambda1 B cell defect in SJL and lambda1-valine knock-in mice with a compensatory increase in lambda2,3 B cells. The defect is due to low signaling by the lambda1-valine BCR. lambda1-Valine B cells deficient in the SHP-1 phosphatase survive better than lambda2,3 B cells in these mice, or lambda1 B cells in lambda1 wildtype mice. Low signaling is apparently due to misfolding of the lambda1-valine light chain as demonstrated by the absence of a regular beta-sheet structure determined by circular dichroism, the sedimentation of the light chain in solution, and the association of valine-valine constant regions in a yeast two-hybrid assay. lambda1-Valine B cells that survive apparently have a higher BCR signal, presumably because of their specific lambda1-heavy chain combination or having encountered a high-affiniy antigen. lambda1-Valine mice have increased B1 cells which were shown by others to have a higher signaling potential. Valine mice crossed with non-conventional gamma2b transgenic mice, in which B cell development is accelerated and in which B1 cells and high signaling cells are greatly reduced, have essentially no, lambda2,3 B cells, but increased numbers of lambda1-valine B cells. This supports the conclusion that the major defect in lambda1-valine mice is the inability of valine-preB cells to produce a threshold signal for B cell development. The reduction of lambda2,3 B cells in valine mice with a gamma2b transgene shows that the majority of their compensatory increase is almost entirely of the B1 cell type.

Molecular basis for the Cu2+ binding-induced destabilization of beta2-microglobulin revealed by molecular dynamics simulation

According to experimental data, binding of the Cu(2+) ions destabilizes the native state of beta2-microglobulin (beta2m). The partial unfolding of the protein was generally considered an early step toward fibril formation in dialysis-related amyloidosis. Recent NMR studies have suggested that the destabilization of the protein might be achieved through increased flexibility upon Cu(2+) binding. However, the molecular mechanism of destabilization due to Cu(2+), its role in amyloid formation, and the relative contributions of different potential copper-binding sites remain unclear. To elucidate the effect of ion ligation at atomic detail, a series of molecular dynamics simulations were carried out on apo- and Cu(2+)-beta2m systems in explicit aqueous solutions, with varying numbers of bound ions. Simulations at elevated temperatures (360 K) provide detailed pictures for the process of Cu(2+)-binding-induced destabilization of the native structure at the nanosecond timescale, which are in agreement with experiments. Conformational transitions toward partially unfolded states were observed in protein solutions containing bound copper ions at His-31 and His-51, which is marked by an increase in the protein vibrational entropy, with TDeltaS(vibr) ranging from 30 to 69 kcal/mol. The binding of Cu(2+) perturbs the secondary structure and the hydrogen bonding pattern disrupts the native hydrophobic contacts in the neighboring segments, which include the beta-strand D2 and part of the beta-strand E, B, and C and results in greater exposure of the D-E loop and the B-C loop to the water environment. Analysis of the MD trajectories suggests that the changes in the hydrophobic environment near the copper-binding sites lower the barrier of conformational transition and stabilize the more disordered conformation. The results also indicate that the binding of Cu(2+) at His-13 has little effect on the conformational stability, whereas the copper-binding site His-31, and to a lesser extent His-51, are primarily responsible for the observed changes in the protein conformation and dynamics.

Where disease pathogenesis meets protein formulation: Renal deposition of immunoglobulin aggregates

Aggregation is one of the important issues encountered during the development of immunoglobulin-based drugs. The aim of the current review is to discuss the causes and consequences of immunoglobulin aggregation as well as the relevance of immunoglobulin aggregation to disease pathogenesis. Extracellular deposition of immunoglobulins, either monoclonal light chains or intact polyclonal antibodies, induces renal failure in various nephropathies. The aggregates can present fibrillar or amorphous structures. In this review, factors known to influence protein aggregation, such as the primary structure of the protein, local environment and glycosylation are assessed, as well as the subsequent altered clearance, fibril formation and toxicity. The role of the protein local environment is emphasized. Even if the local environment causes only minor perturbations in the protein structure, these perturbations might be sufficient to trigger aggregate formation. This fact underlines the importance of choosing appropriate formulations for protein drugs. If the formulation provides a slightly destabilizing environment to the protein, the long-term stability of the drug may be compromised by aggregate formation.

From chance to frequent encounters: Origins of β2-microglobulin fibrillogenesis

It is generally accepted that amyloid formation requires partial, but not complete unfolding of a polypeptide chain. Amyloid formation by beta-2 microglobulin (beta2m), however, readily occurs under strongly native conditions provided that there is exposure to specific transition metal cations. In this review, we discuss transition metal catalyzed conformational changes in several amyloidogenic systems including prion protein, Alzheimer's and Parkinson's diseases. For some systems, including beta2m from dialysis related amyloidosis (DRA), catalysis overcomes an entropic barrier to protein aggregation. Recent data suggest that beta2m samples conformations that are under thermodynamic control, resulting in local or partial unfolding under native conditions. Furthermore, exposure to transition metal cations stabilizes these partially unfolded states and promotes the formation of small oligomers, whose structures are simultaneously near-native and amyloid-like. By serving as a tether, Cu(2+) enables the encounter of amyloidogenic conformations to occur on time scales which are significantly more rapid than would occur between freely diffusing monomeric protein. Once amyloid formation occurs, the requirement for Cu(2+) is lost. We assert that beta2m amyloid fiber formation at neutral pH may be facilitated by rearrangements catalyzed by the transient and pair wise tethering of beta2m at the blood/dialysate interface present during therapeutic hemodialysis.

Human Immunoglobulin Light Chains λ Form Amyloid Fibrils and Granular Aggregates in Solution

Myeloma nephropathy is a disorder characterized by deposition of monoclonal immunoglobulin light chains in the kidneys. The chains deposited form either amyloid fibrils or granular (amorphous) aggregates. Distinct molecular mechanisms leading to the formation of different aggregate types in kidney of patients with multiple myeloma are poorly understood. Here we describe the self-association kinetics of human monoclonal immunoglobulin light chains lambda (GRY) isolated from urine of a patient with multiple myeloma. Under physiological conditions, the isolated light chain exists predominantly in a form of covalent dimer with apparent molecular mass of 50.1 kD. Spectral probe binding, analytical gel filtration, Western blot analysis, and electron microscopy indicate that GRY dimer aggregation occurs via two different pathways producing either amyloid fibrils or amorphous aggregates depending on microenvironment. Incubation of GRY (25 microM) for 4-14 days at 37 degrees C in phosphate buffered saline (PBS), pH 7.0, or in PBS containing urea (0.8 M), pH 6.5, leads to amyloid fibril formation. Under electron microscopy, the fibrils show unbranched thread-like structures, approximately 60-80 x 1000 A in size, which can bind thioflavin T and Congo Red. GRY maintained in acetate buffer, pH 3.5, forms granular aggregates. The structure of GRY oligomers formed during the early stage of amyloid fibril formation (1-4 days) has been examined by means of protein cross-linking with homobifunctional reagents. These oligomers are predominantly trimers and tetramers.

Oligomeric Assembly of Native-like Precursors Precedes Amyloid Formation by β-2 Microglobulin

The deposition of beta-2-microglobulin (beta2m) as amyloid fibers results in debilitating complications for renal failure patients who are treated by hemodialysis. In vitro, wild-type beta2m can be converted to amyloid under physiological conditions by exposure to biomedically relevant concentrations of Cu(2+). In this work, we have made comparative measurements of the structural and oligomeric changes in beta2m at time points preceding fibrillogenesis. Our results show Cu(2+) mediates the formation of a monomeric, activated state followed by the formation of a discrete dimeric intermediate. The dimeric intermediates then assemble into tetra- and hexameric forms which display little additional oligomerization on the time scales of their own formation (<1 h). Amyloid fiber formation progresses from these intermediate states but on much longer time scales (>1 week). Although Cu(2+) is necessary for the generation and stabilization of these intermediates, it is not required for the stability of mature amyloid fibers. This suggests that Cu(2+) acts as an initiating factor of amyloidosis by inducing oligomer formation. (1)H NMR and near-UV circular dichroism are used to establish that oligomeric intermediates are native-like in structure. The native-like structure and discrete oligomeric size of beta2m amyloid intermediates suggest that this protein forms fibrils by structural domain swapping.

Increase in the conformational flexibility of beta 2-microglobulin upon copper binding: a possible role for copper in dialysis-related amyloidosis

A key pathological event in dialysis-related amyloidosis is the fibril formation of beta(2)-microglobulin (beta 2-m). Because beta 2-m does not form fibrils in vitro, except under acidic conditions, predisposing factors that may drive fibril formation at physiological pH have been the focus of much attention. One factor that may be implicated is Cu(2+) binding, which destabilizes the native state of beta 2-m and thus stabilizes the amyloid precursor. To address the Cu(2+)-induced destabilization of beta 2-m at the atomic level, we studied changes in the conformational dynamics of beta 2-m upon Cu(2+) binding. Titration of beta 2-m with Cu(2+) monitored by heteronuclear NMR showed that three out of four histidines (His13, His31, and His51) are involved in the binding at pH 7.0. (1)H-(15)N heteronuclear NOE suggested increased backbone dynamics for the residues Val49 to Ser55, implying that the Cu(2+) binding at His51 increased the local dynamics of beta-strand D. Hydrogen/deuterium exchange of amide protons showed increased flexibility of the core residues upon Cu(2+) binding. Taken together, it is likely that Cu(2+) binding increases the pico- to nanosecond fluctuation of the beta-strand D on which His51 exists, which is propagated to the core of the molecule, thus promoting the global and slow fluctuations. This may contribute to the overall destabilization of the molecule, increasing the equilibrium population of the amyloidogenic intermediate.

Cu2+-induced modification of the kinetics of A beta(1-42) channels

We found that the amyloid beta peptide A beta(1-42) is capable of interacting with membrane and forming heterogeneous ion channels in the absence of any added Cu2+ or biological redox agents that have been reported to mediate A beta(1-42) toxicity. The A beta(1-42)-formed cation channel was inhibited by Cu2+ in cis solution ([Cu2+]cis) in a voltage- and concentration-dependent manner between 0 and 250 microM. The [Cu2+]cis-induced channel inhibition is fully reversible at low concentrations between 50 and 100 microM [Cu2+]cis and partially reversible at 250 microM [Cu2+]cis. The inhibitory effects of [Cu2+]cis between 50 and 250 microM on the channel could not be reversed with addition of Cu2+-chelating agent clioquinol (CQ) at concentrations between 64 and 384 microM applied to the cis chamber. The effects of 200-250 microM [Cu2+]cis on the burst and intraburst kinetic parameters were not fully reversible with either wash or 128 microM [CQ]cis. The kinetic analysis of the data indicate that Cu2+-induced inhibition was mediated via both desensitization and an open channel block mechanism and that Cu2+ binds to the histidine residues located at the mouth of the channel. It is proposed that the Cu2+-binding site of the A beta(1-42)-formed channels is modulated with Cu2+ in a similar way to those of channels formed with the prion protein fragment PrP(106-126), suggesting a possible common mechanism for Cu2+ modulation of A beta and PrP channel proteins linked to neurodegenerative diseases.

A systematic investigation into the effect of protein destabilisation on beta 2-microglobulin amyloid formation

Beta-2-microglobulin (beta(2)m) has been shown to form amyloid fibrils with distinct morphologies under acidic conditions in vitro. Short, curved fibrils (<600 nm in length), form rapidly without a lag phase, with a maximum rate at pH 3.5. By contrast, fibrils with a long (approximately 1 microm), straight morphology are produced by incubation of the protein at pH< or =3.0. Both fibril types display Congo red birefringence, bind Thioflavin-T and have X-ray fibre diffraction patterns consistent with a cross-beta structure. In order to investigate the role of different partially folded states in generating fibrils of each type, and to probe the effect of protein stability on amyloid formation, we have undertaken a detailed mutagenesis study of beta(2)m. Thirteen variants containing point mutations in different regions of the native protein were created and their structure, stability and fibril forming propensities were investigated as a function of pH. By altering the stability of the native protein in this manner, we show that whilst destabilisation of the native state is important in the generation of amyloid fibrils, population of specific denatured states is a pre-requisite for amyloid formation from this protein. Moreover, we demonstrate that the formation of fibrils with different morphologies in vitro correlates with the relative population of different precursor states.

A model for amyloid fibril formation in immunoglobulin light chains based on comparison of amyloidogenic and benign proteins and specific antibody binding

In an attempt to understand the mechanism of amyloid fibril formation in light chain amyloidosis, the properties of amyloidogenic (SMA) and benign (LEN) immunoglobulin light chain variable domains (VL) were compared. The conformations of LEN and SMA were measured using secondary and tertiary structural probes over the pH range from 2 and 8. At all pH values, LEN was more stable than SMA. The CD spectra of LEN at pH 2 were comparable to those of SMA at pH 7.5, indicating that the low pH conformation of LEN closely resembles that of SMA at physiological pH. At low pH, a relatively unfolded intermediate conformation is populated for SMA and rapidly leads to amyloid fibrils. The lack of such an intermediate with LEN, is attributed to sequence differences and accounts for the lack of LEN fibrils in the absence of agitation. A kappa IV-specific monoclonal antibody that recognizes the N-terminal of SMA caused unraveling of the fibrils to the protofilaments and was observed to bind to one end of SMA protofilaments by atomic force microscopy. The antibody result indicates that each protofilament is asymmetric with different ends. A model for the formation of fibrils by SMA is proposed.

Formation of a copper specific binding site in non-native states of beta-2-microglobulin

A debilitating complication of long-term hemodialysis is the deposition of beta-2-microglobulin (beta2m) as amyloid plaques in the joint space. We have recently shown that Cu(2+) can be a contributing, if not causal, factor at concentrations encountered during dialysis therapy. The basis for this effect is destabilization and incorporation of beta2m into amyloid fibers upon binding of Cu(2+). In this work, we demonstrate that while beta2m binds Cu(2+) specifically in the native state, it is binding of Cu(2+) by non-native states of beta2m which is responsible for destabilization. Mutagenesis of potential coordinating groups for Cu(2+) shows that native state binding of Cu(2+) is mediated by residues and structures that are different than those which bind in non-native states. An increased affinity for copper by non-native states compared to that of the native state gives rise to overall destabilization. Using mass spectrometry, NMR, and fluorescence techniques, we show that native state binding is localized to H31 and W60 and is highly specific for Cu(2+) over Zn(2+) and Ni(2+). Binding of Cu(2+) in non-native states of beta2m is mediated by residues H13, H51, and H84, but not H31. Although denatured beta2m has characteristics of a globally unfolded state, it nevertheless demonstrates the following strong specificity of binding: Cu(2+) > Zn(2+) >> Ni(2+). This requires the existence of a well-defined structure in the unfolded state of this protein. As Cu(2+) effects are reported in many other amyloidoses, e.g., PrP, alpha-synuclein, and Abeta, our results may be extended to the emerging field of divalent ion-associated amyloidosis.